A two-dimensional discrete dislocation dynamics （DDD） technology by Giessen and Needleman （1995）, which has been extended by integrating a dislocation-grain boundary interaction model, is used to computationally a...A two-dimensional discrete dislocation dynamics （DDD） technology by Giessen and Needleman （1995）, which has been extended by integrating a dislocation-grain boundary interaction model, is used to computationally analyze the micro-cyclic plastic response of polycrystals containing micron-sized grains, with special attentions to significant influence of dislocationpenetrable grain boundaries （GBs） on the micro-plastic cyclic responses of polycrystals and underlying dislocation mechanism. Toward this end, a typical polycrystalline rectangular specimen under simple tension-compression loading is considered. Results show that, with the increase of cycle accumulative strain, continual dislocation accumulation and enhanced dislocation-dislocation interactions induce the cyclic hardening behavior; however, when a dynamic balance among dislocation nucleation, penetration through GB and dislocation annihilation is approximately established, cyclic stress gradually tends to saturate. In addition, other factors, including the grain size, cyclic strain amplitude and its history, also have considerable influences on the cyclic hardening and saturation.展开更多
Preparing method and processing of Tb-Dy-Fe alloy samples with [110] axial orientation as well as their magnetostrictive properties have been studied. It has been found that the magnetostrictive strains of polycrystal...Preparing method and processing of Tb-Dy-Fe alloy samples with [110] axial orientation as well as their magnetostrictive properties have been studied. It has been found that the magnetostrictive strains of polycrystal samples with [110] axial orientation can reach (1550-1900) ×10~-6 in a low magnetic field less than 80 kA/m, which are equal to or somewhat better than that of the polycrystal samples with [112] axial orientation.展开更多
Some physical properties of crystals differ in direction n because crystal lattices are often anisotropic. A polycrystal is an aggregate of numerous tiny crystallites. Unless the polycrystal is an isotropic aggregate ...Some physical properties of crystals differ in direction n because crystal lattices are often anisotropic. A polycrystal is an aggregate of numerous tiny crystallites. Unless the polycrystal is an isotropic aggregate of crystallites, the physical properties of the polycrystal vary with n. The direction-dependent functions （DDF） for crystals and polycrystals are introduced to describe the variations of the physical properties in direction n. Until now there are few papers dealing systematically with relations between the DDF and the crystalline orientation distribution. Herein we give general expressions of the DDF for crystals and polycrystals. We discuss the applications of the DDF in describing the physical properties of crystals and polycrystals.展开更多
A finite element formulation which derives constitutive response from crystal plasticity theory was used to examine localized deformation in fee polycrystals. The polycrystal model was an idealized planar array of 22 ...A finite element formulation which derives constitutive response from crystal plasticity theory was used to examine localized deformation in fee polycrystals. The polycrystal model was an idealized planar array of 22 hexagonal grains. The constitutive description used is based on a finite strain kinematical theory that accounts for lattice rotations. Formation of shear bands was successfully modeled in both single crystal and polycrystals. Stress and strain distribution around triple junctions was also analyzed. Results show the distributions of stresses and strains are distinctly inhomogeneous. Stress and strain fields across grain boundaries are highly discontinuous. However, this discontinuity will be restrained when shear bands are fully developed.展开更多
Creep behaviour of Mo and Fe polycrystals has been studied for a wide range of Strain rates in the presence of grain-boundary diffusion flux of Ni. The role of the impurity grain-boundary diffusion and grain-boundary ...Creep behaviour of Mo and Fe polycrystals has been studied for a wide range of Strain rates in the presence of grain-boundary diffusion flux of Ni. The role of the impurity grain-boundary diffusion and grain-boundary migratin in the realization of the superplastic strain of metals under these conditions is examined展开更多
By the nonlinear optimization theory, we predict the yield function of single BCC crystals in Hill＇s criterion form. Then we give a formula on the macroscopic yield function of a BCC polycrystal Ω under Sachs＇ mode...By the nonlinear optimization theory, we predict the yield function of single BCC crystals in Hill＇s criterion form. Then we give a formula on the macroscopic yield function of a BCC polycrystal Ω under Sachs＇ model, where the volume average of the yield functions of all BCC crystallites in Ω is taken as the macroscopic yield function of the BCC polycrystal. In constructing the formula, we try to find the relationship among the macroscopic yield function, the orientation distribution function （ODF）, and the single BCC crystal＇s plasticity. An expression for the yield stress of a uniaxial tensile problem is derived under Taylor＇s model in order to compare the expression with that of the macroscopic yield function.展开更多
In the epitaxial growth of two-dimensional(2D)materials on substrates,2D polycrystals with various shapes have been broadly observed,but their formation mechanisms are still highly elusive.Here we present a complete s...In the epitaxial growth of two-dimensional(2D)materials on substrates,2D polycrystals with various shapes have been broadly observed,but their formation mechanisms are still highly elusive.Here we present a complete study on the formation mechanisms of various 2D polycrystals.The structures of the 2D polycrystals are dependent on the symmetries of both the 2D material and the substrate.We build four complete libraries of 2D polycrystals for(i)threefold symmetric 2D materials on two-or six-fold symmetric substrates(i.e.,family-Ⅲ/Ⅱor-Ⅲ/Ⅵ),(ii)threefold symmetric 2D materials on fourfold symmetric substrates(i.e.family-Ⅲ/Ⅳ),(iii)fourfold symmetric 2D materials on three-or six-fold symmetric substrates(i.e.,family-Ⅳ/Ⅲor-Ⅳ/Ⅵ),and(iv)sixfold symmetric 2D materials on fourfold symmetric substrates(i.e.,family-Ⅵ/Ⅳ),respectively.The four libraries of 2D polycrystals are consistent with many existing experimental observations and can be used to guide the experimental synthesis of various 2D polycrystals.展开更多
Crystallographic texturing enables the design of piezoelectric polycrystals that outperform traditional random polycrystals by exhibiting outstanding piezoelectric properties.In this work,phase-field modeling and comp...Crystallographic texturing enables the design of piezoelectric polycrystals that outperform traditional random polycrystals by exhibiting outstanding piezoelectric properties.In this work,phase-field modeling and computer simulation were employed to study the effect of crystallographic texture on the piezoelectric properties of ferroelectric polycrystals at the domain level.Domain evolutions for single crystal,random polycrystal,and textured polycrystal are systematically simulated.The simulations reveal that the[001]-textured polycrystal can fully exploit the intrinsic anisotropic properties of piezoelectric materials by exhibiting a piezoelectric coefficient that is as large as that of single crystal while being much larger than that of random polycrystal.To better understand the mechanism of piezoelectricity enhancement by crystallographic texturing,a theoretical analysis based on Landau theory is provided.In comparison with random polycrystal,the textured polycrystal manifests a flatter energy landscape and thus possesses a higher piezoelectric coefficient.展开更多
Hydrogen as an interstitial solute at grain boundaries(GBs)can have a catastrophic impact on the mechanical properties of many metals.Despite the global research effort,the underlying hydrogen-GB interactions in polyc...Hydrogen as an interstitial solute at grain boundaries(GBs)can have a catastrophic impact on the mechanical properties of many metals.Despite the global research effort,the underlying hydrogen-GB interactions in polycrystals remain inadequately understood.In this study,using Voronoi tessellations and atomistic simulations,we elucidate the hydrogen segregation energy spectrum at the GBs of polycrystalline nickel by exploring all the topologically favorable segregation sites.Three distinct peaks in the energy spectrum are identified,corresponding to different structural fingerprints.The first peak(-0.205 eV)represents the most favorable segregation sites at GB core,while the second and third peaks account for the sites at GB surface.By incorporating a thermodynamic model,the spectrum enables the determination of the equilibrium hydrogen concentrations at GBs,unveiling a remarkable two to three orders of magnitude increase compared to the bulk hydrogen concentration reported in experimental studies.The identified structures from the GB spectrum exhibit vastly different behaviors in hydrogen segregation and diffusion,with the low-barrier channels inside GB core contributing to short-circuit diffusion,while the high energy gaps between GB and neighboring lattice serving as on-plane diffusion barriers.Mean square displacement analysis further confirms the findings,and shows that the calculated GB diffusion coefficient is three orders of magnitude greater than that of lattice.The present study has a significant implication for practical applications since it offers a tool to bridge the gap between atomic-scale interactions and macroscopic behaviors in engineering materials.展开更多
A new polycrystal model was presented from the viewpoint of polycrystal structure of the billets considering free surface effects.In the model,the billet was divided into three portions,such as free surface portion,tr...A new polycrystal model was presented from the viewpoint of polycrystal structure of the billets considering free surface effects.In the model,the billet was divided into three portions,such as free surface portion,transition portion and internal portion.The grains in free surface portion were considered the single grains,and the anisotropy of the grains was taken into account by introducing grain orientation to explain the inhomogeneous deformation.In the transition portion,the effects of the neighbouring grains were adopted in the model.The grains in the internal portion were considered the polycrystalline material.With the developed model,the upsetting deformation process was simulated by the MSC Superform software.The scatter of the flow stress and inhomogeneous deformation was observed by analysis of the model.The comparisons show that the computational results are good agreed with the experimental results.This means that the presented model is effective.展开更多
A polycrystal plasticity model was developed to analyze the room-temperature deformation behaviors of Mg-3A1-1Zn alloy（AZ31）.The uniaxial tension and compression tests at room temperature were conducted using cast a...A polycrystal plasticity model was developed to analyze the room-temperature deformation behaviors of Mg-3A1-1Zn alloy（AZ31）.The uniaxial tension and compression tests at room temperature were conducted using cast and extruded AZ31 rods with different textures and combined with the proposed model to reveal the deformation mechanisms.It is shown that,different flow curves of two specimens under tension and compression tests can be simulated by this model.The flow curves of AZ31 extrusions exhibit different shapes for tension and compression due to different activities of tensile twinning and pyramidalc＋a slip.The metallographic and TEM observations showed the equal twinning activities at the initial stage in tension and compression tests and the occurrence of pyramidalc＋a slip in compression of as-cast Mg-3A1-1Zn alloy with increasing the strain,which is consistent with the simulated results by the proposed model.展开更多
Fe80Ga20-xAlx （x = 0, 6, 9, 14） ingots were prepared from high purity elements using a vacuum induction system. X-ray diffraction patterns show that the alloys are A2 disordered structures. The influence of the part...Fe80Ga20-xAlx （x = 0, 6, 9, 14） ingots were prepared from high purity elements using a vacuum induction system. X-ray diffraction patterns show that the alloys are A2 disordered structures. The influence of the partial substitution of Ga in Fe-Ga alloys with A1 on their magnetostrictive properties was investigated, and the effects of different heat treatment conditions on the magnetostriction and microstructure of the alloy rods were also examined. The saturation magnetostriction value of FesoGa2o can reach to 240 x 10-6 under a compressive stress of 20 MPa. The Fe80GallA19 alloy has many good properties, such as low hysteresis, high linearity of the magnetostriction curve, and low saturated magnetic field, which make it a potential candidate for magnetostrictive actuator and transducer applications. It is found that subgrains have little influence on the magnetostriction of Fe-Ga alloys.展开更多
Co1-x-yNix＋ySb3-xSnx polycrystals were fabricated by vacuum melting combined with hot-press sintering. The effect of alloying on the thermoelectric properties of unfilled skutterudite CO1-xNixSb3-xSnx was investigate...Co1-x-yNix＋ySb3-xSnx polycrystals were fabricated by vacuum melting combined with hot-press sintering. The effect of alloying on the thermoelectric properties of unfilled skutterudite CO1-xNixSb3-xSnx was investigated. A leap of electrical conductivity from the Co0.93Ni0.07Sb2.93Sn0.07 sample to the Co0.88Ni0.12Sb288Sn0.12 sample occurs during the measurement of electrical conductivity, indicating the adjustment of band structure by proper alloying. The results show that alloying enhances the power factor of the materials. On the basis of alloying, the thermoelectric properties of Coo.88Nio.12Sb2.ssSno.12 are improved by Ni-doping. The thermal conductivities of Ni-doping samples have no reduction, but their power factors have obvious enhancement. The power factor of Co0.81Ni0.09Sb2.88Sn0.12 reaches 3.0 mW-m-1·K-2 by Ni doping. The dimensionless thermoelectric figure of merit reaches 0.55 at 773 K for the unfilled Co0.81Ni0.19 Sb2.88Sn0.12,展开更多
A systematic study has been conducted aiming to attain an insight into the influence of coefficient of roll speed asymmetry, crystal orientation and structure on the deformation behavior, and crystallographic orientat...A systematic study has been conducted aiming to attain an insight into the influence of coefficient of roll speed asymmetry, crystal orientation and structure on the deformation behavior, and crystallographic orientation development during foil rolling. Simulations were successfully carried out by using crystal plasticity finite element method（CPFEM）,and a novel computational framework is presented for the representation of virtual polycrystalline grain structures. It has been found that asymmetric rolling（ASR） is more efficient in producing plastic deformation since it develops additional shear strain and activity of slip system compared with symmetric rolling（SR）. For ASR, increase in the length of the shear zone, and decrease in the amount of the pressure and roll force would lead to further reduction. The shear strain path in SR and ASR is strictly influenced by the misorientation of neighbor grains, and corresponding {1 1 1} pole figures offer direct evidence of the spread of crystallographic orientation around the normal direction. The activity of slip systems was examined in detail and found that the predicted results are consistent with the surface layer model. The accuracy of the developed CPFEM model is verified by the fact that the simulated results of roll force coincide well with the experimental results.展开更多
The microstructures of Si substrate/amorphous SiO2 layer/SOI Si layer / polycrystal Si layer / amorphous SiO2 layer / coarse grain Al electrode were studied by using cross-section transmission electron microscopy, sca...The microstructures of Si substrate/amorphous SiO2 layer/SOI Si layer / polycrystal Si layer / amorphous SiO2 layer / coarse grain Al electrode were studied by using cross-section transmission electron microscopy, scanning electron microscopy, and high resolution electron microscopy. The straightness of interface between amorphous SiO2 layer and Si substrate is in 2-3 atomic layer. The defects were seldom found in the active area of SOI Si layer, and can be effectively confined in defect entrainment regions. The defects observed in defect entrainment regions are subgrain boundaries, large angle boundaries and dislocations. The dislocation loops which are parallel to the interface and have images with lines of no contrast under two beam condition have been observed in the side of SOI Si layer. Polycrystal Si layer consists of columnar and fine grains without preferential orientation. An ultra-thin amorphous SiO2 layer exists between SOI Si layer and polycrystal Si layer.展开更多
It is pointed out that crystals are discrete but not continuous materials. Hence the rotation R in decomposition F = RU and spin TY in (F) over dot F-1 ore not correct. Errors will arise in plastic deformation rare if...It is pointed out that crystals are discrete but not continuous materials. Hence the rotation R in decomposition F = RU and spin TY in (F) over dot F-1 ore not correct. Errors will arise in plastic deformation rare if it is directly expressed with amounts of velocity of slips in glide systems such as (gamma) over dot upsilon circle times n. The geometrical figure of crystal lattices does nor change after slips and based on this idea a simple way in mechanics of continuous media to get the plastic deformations rare induced by slips is proposed. Constitutive equations are recommended.展开更多
In this paper,the dependencies of Young's modulus and attenuation decrement on samarium sulfide polycrystals(SmS)under various annealing temperatures are studied by the piezoelectric ultrasonic composite oscillato...In this paper,the dependencies of Young's modulus and attenuation decrement on samarium sulfide polycrystals(SmS)under various annealing temperatures are studied by the piezoelectric ultrasonic composite oscillator technique at a frequency of 100 kHz in the temperature range of 80-300 K.A decrease in Young's modulus with an increase of the annealing temperature due to the texturing of the material was revealed.At the same time,attenuation peaks were observed at temperatures about 90 and 125 K,presumably due to Niblett-Wilks and Bordoni relaxations.展开更多
The visco-plastic self-consistent (VPSC) model is extended to take the dynamical recrystallization (DRX) into account so that the hot extrusion texture of AZ80 magnesium alloy can be properly modeled. The effects of e...The visco-plastic self-consistent (VPSC) model is extended to take the dynamical recrystallization (DRX) into account so that the hot extrusion texture of AZ80 magnesium alloy can be properly modeled. The effects of extrusion temperatures and imposed boundary conditions on the resulting textures were investigated, and good agreement can be found between the simulated and the measured extrusion textures. The simulated results show that the DRX grains are responsible for the formation of the {2110} fiber component since the {1010} poles of the DRX grains are tilted away from those of the unrecrystallized grains during the formation of their high angle boundaries (HABs). Furthermore, the basal poles of the grains are favorably oriented to the transversal direction (TD) where the imposed deformation is larger due to lower slip resistance of the basal slip. The elevated temperature enhances the activity of pyramidal ?c+a? slip modes and gives rise to a larger recrystallized volume fraction, resulting in a weakened extrusion texture.展开更多
The aim of present work is to develop a crystal plasticity modeling approach to integrate slip,dynamic recrystallization(DRX)and grain boundary sliding(GBS)for simulating the deformation behavior and texture evolution...The aim of present work is to develop a crystal plasticity modeling approach to integrate slip,dynamic recrystallization(DRX)and grain boundary sliding(GBS)for simulating the deformation behavior and texture evolution of magnesium alloys at high temperatures.Firstly,the deformation mechanisms of an AZ31B Mg alloy sheet at 300°C were investigated by examining texture and microstructure evolution during uniaxial tension and compression tests.DRX refines microstructure at strains less than 0.2,and subsequently GBS plays a significant role during deformation process.A GBS model is developed to evaluate strain and grain rotation induced by GBS,and implemented into the polycrystal plasticity framework VPSC.The VPSC-DRX-GBS model can well reproduce the stress−strain curves,grain size,texture evolution and significant texture differences in tension and compression tests due to GBS.The calculated GBS contribution ratio in tension is obviously higher than that in compression due to easier cavity nucleation at grain boundaries under tension loading.展开更多
An orthorhombic polycrystal is an orthorhombic aggregate of tiny crystallites. In this paper, we study the effect of the crystalline mean shape on the constitutive relation of the orthorhombic polycrystal. The crystal...An orthorhombic polycrystal is an orthorhombic aggregate of tiny crystallites. In this paper, we study the effect of the crystalline mean shape on the constitutive relation of the orthorhombic polycrystal. The crystalline mean shape and the crystalline orientation arrangement are described by the crystalline shape function （CSF） and the orientation distribution function （ODF）, respectively. The CSF and the ODF are expanded as an infinite series in terms of the Wigner D-functions. The expanded coefficients of the CSF and the ODF are called the shape coefficients s^lm0 and the texture coefficients c^lmn respectively. Assuming that Ceff in the constitutive relation depends on the shape coefficients s^lm0 and the texture coefficients c^lmn by the principle of material frame-indifference we derive an analytical expression for C^eff up to terms linear in s^lmo and c^lmn and the expression would be applicable to the polycrystal whose texture is weak and whose crystalline mean shape has weak anisotropy. C^cff contains six unspecified material constants （λ, μ, c, s1, s2, s3）, five shape coefficients （s^2 00, s^2 20, s^4 00, s^4 20, s^4 40）, and three texture coefficients （c^4 99,c^4 20, c^4 40）, The results based on the perturbation approach are used to determine the five material constants approximately. We also find that the shape coefficients 2 and a s^2mo and s^4m0 are all zero if the crystalline mean shape is a cuboid. Some examples are given to compare our computational results.展开更多
基金supported by the National Natural Science Foundation of China(No.10672064).
文摘A two-dimensional discrete dislocation dynamics （DDD） technology by Giessen and Needleman （1995）, which has been extended by integrating a dislocation-grain boundary interaction model, is used to computationally analyze the micro-cyclic plastic response of polycrystals containing micron-sized grains, with special attentions to significant influence of dislocationpenetrable grain boundaries （GBs） on the micro-plastic cyclic responses of polycrystals and underlying dislocation mechanism. Toward this end, a typical polycrystalline rectangular specimen under simple tension-compression loading is considered. Results show that, with the increase of cycle accumulative strain, continual dislocation accumulation and enhanced dislocation-dislocation interactions induce the cyclic hardening behavior; however, when a dynamic balance among dislocation nucleation, penetration through GB and dislocation annihilation is approximately established, cyclic stress gradually tends to saturate. In addition, other factors, including the grain size, cyclic strain amplitude and its history, also have considerable influences on the cyclic hardening and saturation.
文摘Preparing method and processing of Tb-Dy-Fe alloy samples with [110] axial orientation as well as their magnetostrictive properties have been studied. It has been found that the magnetostrictive strains of polycrystal samples with [110] axial orientation can reach (1550-1900) ×10~-6 in a low magnetic field less than 80 kA/m, which are equal to or somewhat better than that of the polycrystal samples with [112] axial orientation.
基金supported by the National Natural Science Foundation of China (10562004, 10662004)the Jiangxi Project to Nurture Academic and Technical Leaders in Targeted Areas+1 种基金the Research Fund for the Docotoral Program of Higher Education (20070403003)the Natural Science Foundation of Jiangxi of China (2008GZW0005).
文摘Some physical properties of crystals differ in direction n because crystal lattices are often anisotropic. A polycrystal is an aggregate of numerous tiny crystallites. Unless the polycrystal is an isotropic aggregate of crystallites, the physical properties of the polycrystal vary with n. The direction-dependent functions （DDF） for crystals and polycrystals are introduced to describe the variations of the physical properties in direction n. Until now there are few papers dealing systematically with relations between the DDF and the crystalline orientation distribution. Herein we give general expressions of the DDF for crystals and polycrystals. We discuss the applications of the DDF in describing the physical properties of crystals and polycrystals.
文摘A finite element formulation which derives constitutive response from crystal plasticity theory was used to examine localized deformation in fee polycrystals. The polycrystal model was an idealized planar array of 22 hexagonal grains. The constitutive description used is based on a finite strain kinematical theory that accounts for lattice rotations. Formation of shear bands was successfully modeled in both single crystal and polycrystals. Stress and strain distribution around triple junctions was also analyzed. Results show the distributions of stresses and strains are distinctly inhomogeneous. Stress and strain fields across grain boundaries are highly discontinuous. However, this discontinuity will be restrained when shear bands are fully developed.
文摘Creep behaviour of Mo and Fe polycrystals has been studied for a wide range of Strain rates in the presence of grain-boundary diffusion flux of Ni. The role of the impurity grain-boundary diffusion and grain-boundary migratin in the realization of the superplastic strain of metals under these conditions is examined
基金Project supported by the National Natural Science Foundation of China (No. 10562004) the Natural Science Foundation of Jiangxi (Nos.0450035 and 0512021) the Science Foundation of Jiangxi Educational Department (No.[2006]3) the Oversea Returned Scholars Grant of China.
文摘By the nonlinear optimization theory, we predict the yield function of single BCC crystals in Hill＇s criterion form. Then we give a formula on the macroscopic yield function of a BCC polycrystal Ω under Sachs＇ model, where the volume average of the yield functions of all BCC crystallites in Ω is taken as the macroscopic yield function of the BCC polycrystal. In constructing the formula, we try to find the relationship among the macroscopic yield function, the orientation distribution function （ODF）, and the single BCC crystal＇s plasticity. An expression for the yield stress of a uniaxial tensile problem is derived under Taylor＇s model in order to compare the expression with that of the macroscopic yield function.
基金The authors acknowledge support from the Institute for Basic Science(IBS-R019-D1)South Korea,the Chinese Academy of Sciences,and the Strategic Priority Research Program of the Chinese Academy of Sciences(XDB30000000),P.R.China.
文摘In the epitaxial growth of two-dimensional(2D)materials on substrates,2D polycrystals with various shapes have been broadly observed,but their formation mechanisms are still highly elusive.Here we present a complete study on the formation mechanisms of various 2D polycrystals.The structures of the 2D polycrystals are dependent on the symmetries of both the 2D material and the substrate.We build four complete libraries of 2D polycrystals for(i)threefold symmetric 2D materials on two-or six-fold symmetric substrates(i.e.,family-Ⅲ/Ⅱor-Ⅲ/Ⅵ),(ii)threefold symmetric 2D materials on fourfold symmetric substrates(i.e.family-Ⅲ/Ⅳ),(iii)fourfold symmetric 2D materials on three-or six-fold symmetric substrates(i.e.,family-Ⅳ/Ⅲor-Ⅳ/Ⅵ),and(iv)sixfold symmetric 2D materials on fourfold symmetric substrates(i.e.,family-Ⅵ/Ⅳ),respectively.The four libraries of 2D polycrystals are consistent with many existing experimental observations and can be used to guide the experimental synthesis of various 2D polycrystals.
文摘Crystallographic texturing enables the design of piezoelectric polycrystals that outperform traditional random polycrystals by exhibiting outstanding piezoelectric properties.In this work,phase-field modeling and computer simulation were employed to study the effect of crystallographic texture on the piezoelectric properties of ferroelectric polycrystals at the domain level.Domain evolutions for single crystal,random polycrystal,and textured polycrystal are systematically simulated.The simulations reveal that the[001]-textured polycrystal can fully exploit the intrinsic anisotropic properties of piezoelectric materials by exhibiting a piezoelectric coefficient that is as large as that of single crystal while being much larger than that of random polycrystal.To better understand the mechanism of piezoelectricity enhancement by crystallographic texturing,a theoretical analysis based on Landau theory is provided.In comparison with random polycrystal,the textured polycrystal manifests a flatter energy landscape and thus possesses a higher piezoelectric coefficient.
基金financially supported by the Research Council of Norway under the M-HEAT project(No.294689)the HyLINE Project(No.294739)All simulation resources are provided by the Norwegian Metacenter for Computational Science(Nos.NN9110K and NN9391K).
文摘Hydrogen as an interstitial solute at grain boundaries(GBs)can have a catastrophic impact on the mechanical properties of many metals.Despite the global research effort,the underlying hydrogen-GB interactions in polycrystals remain inadequately understood.In this study,using Voronoi tessellations and atomistic simulations,we elucidate the hydrogen segregation energy spectrum at the GBs of polycrystalline nickel by exploring all the topologically favorable segregation sites.Three distinct peaks in the energy spectrum are identified,corresponding to different structural fingerprints.The first peak(-0.205 eV)represents the most favorable segregation sites at GB core,while the second and third peaks account for the sites at GB surface.By incorporating a thermodynamic model,the spectrum enables the determination of the equilibrium hydrogen concentrations at GBs,unveiling a remarkable two to three orders of magnitude increase compared to the bulk hydrogen concentration reported in experimental studies.The identified structures from the GB spectrum exhibit vastly different behaviors in hydrogen segregation and diffusion,with the low-barrier channels inside GB core contributing to short-circuit diffusion,while the high energy gaps between GB and neighboring lattice serving as on-plane diffusion barriers.Mean square displacement analysis further confirms the findings,and shows that the calculated GB diffusion coefficient is three orders of magnitude greater than that of lattice.The present study has a significant implication for practical applications since it offers a tool to bridge the gap between atomic-scale interactions and macroscopic behaviors in engineering materials.
基金Projects(50835002,50805035)support by the National Natural Science Foundation of ChinaProject(QC08C55)supported by the Natural Science Foundation of Heilongjiang Province,ChinaProject(200802131031)supported by the PhD Programs Foundation of Ministry of Education of China for Young Scholars
文摘A new polycrystal model was presented from the viewpoint of polycrystal structure of the billets considering free surface effects.In the model,the billet was divided into three portions,such as free surface portion,transition portion and internal portion.The grains in free surface portion were considered the single grains,and the anisotropy of the grains was taken into account by introducing grain orientation to explain the inhomogeneous deformation.In the transition portion,the effects of the neighbouring grains were adopted in the model.The grains in the internal portion were considered the polycrystalline material.With the developed model,the upsetting deformation process was simulated by the MSC Superform software.The scatter of the flow stress and inhomogeneous deformation was observed by analysis of the model.The comparisons show that the computational results are good agreed with the experimental results.This means that the presented model is effective.
基金Project(51201092)supported by the National Natural Science Foundation of China
文摘A polycrystal plasticity model was developed to analyze the room-temperature deformation behaviors of Mg-3A1-1Zn alloy（AZ31）.The uniaxial tension and compression tests at room temperature were conducted using cast and extruded AZ31 rods with different textures and combined with the proposed model to reveal the deformation mechanisms.It is shown that,different flow curves of two specimens under tension and compression tests can be simulated by this model.The flow curves of AZ31 extrusions exhibit different shapes for tension and compression due to different activities of tensile twinning and pyramidalc＋a slip.The metallographic and TEM observations showed the equal twinning activities at the initial stage in tension and compression tests and the occurrence of pyramidalc＋a slip in compression of as-cast Mg-3A1-1Zn alloy with increasing the strain,which is consistent with the simulated results by the proposed model.
基金financially supported by the Major State Basic Research Development Program of China(No.2011CB606304)the Fundamental Research Funds for Central Universities of China(No.FRF-SD-12-025A)+1 种基金the National Natural Science Foundation of China(No.51371028)the State Key Lab of Advanced Metals and Materials Funds of China(No.2011Z-02)
文摘Fe80Ga20-xAlx （x = 0, 6, 9, 14） ingots were prepared from high purity elements using a vacuum induction system. X-ray diffraction patterns show that the alloys are A2 disordered structures. The influence of the partial substitution of Ga in Fe-Ga alloys with A1 on their magnetostrictive properties was investigated, and the effects of different heat treatment conditions on the magnetostriction and microstructure of the alloy rods were also examined. The saturation magnetostriction value of FesoGa2o can reach to 240 x 10-6 under a compressive stress of 20 MPa. The Fe80GallA19 alloy has many good properties, such as low hysteresis, high linearity of the magnetostriction curve, and low saturated magnetic field, which make it a potential candidate for magnetostrictive actuator and transducer applications. It is found that subgrains have little influence on the magnetostriction of Fe-Ga alloys.
基金financially supported by the National Natural Science Foundation of China (Nos.50801054 and 51072104)the Research Award Fund for Outstanding Young Scientists in Shandong Province,China (No.BS2011CL031)
文摘Co1-x-yNix＋ySb3-xSnx polycrystals were fabricated by vacuum melting combined with hot-press sintering. The effect of alloying on the thermoelectric properties of unfilled skutterudite CO1-xNixSb3-xSnx was investigated. A leap of electrical conductivity from the Co0.93Ni0.07Sb2.93Sn0.07 sample to the Co0.88Ni0.12Sb288Sn0.12 sample occurs during the measurement of electrical conductivity, indicating the adjustment of band structure by proper alloying. The results show that alloying enhances the power factor of the materials. On the basis of alloying, the thermoelectric properties of Coo.88Nio.12Sb2.ssSno.12 are improved by Ni-doping. The thermal conductivities of Ni-doping samples have no reduction, but their power factors have obvious enhancement. The power factor of Co0.81Ni0.09Sb2.88Sn0.12 reaches 3.0 mW-m-1·K-2 by Ni doping. The dimensionless thermoelectric figure of merit reaches 0.55 at 773 K for the unfilled Co0.81Ni0.19 Sb2.88Sn0.12,
基金financially supported by the National Natural Science Foundation of China (Nos. 51374069 and U1460107)
文摘A systematic study has been conducted aiming to attain an insight into the influence of coefficient of roll speed asymmetry, crystal orientation and structure on the deformation behavior, and crystallographic orientation development during foil rolling. Simulations were successfully carried out by using crystal plasticity finite element method（CPFEM）,and a novel computational framework is presented for the representation of virtual polycrystalline grain structures. It has been found that asymmetric rolling（ASR） is more efficient in producing plastic deformation since it develops additional shear strain and activity of slip system compared with symmetric rolling（SR）. For ASR, increase in the length of the shear zone, and decrease in the amount of the pressure and roll force would lead to further reduction. The shear strain path in SR and ASR is strictly influenced by the misorientation of neighbor grains, and corresponding {1 1 1} pole figures offer direct evidence of the spread of crystallographic orientation around the normal direction. The activity of slip systems was examined in detail and found that the predicted results are consistent with the surface layer model. The accuracy of the developed CPFEM model is verified by the fact that the simulated results of roll force coincide well with the experimental results.
文摘The microstructures of Si substrate/amorphous SiO2 layer/SOI Si layer / polycrystal Si layer / amorphous SiO2 layer / coarse grain Al electrode were studied by using cross-section transmission electron microscopy, scanning electron microscopy, and high resolution electron microscopy. The straightness of interface between amorphous SiO2 layer and Si substrate is in 2-3 atomic layer. The defects were seldom found in the active area of SOI Si layer, and can be effectively confined in defect entrainment regions. The defects observed in defect entrainment regions are subgrain boundaries, large angle boundaries and dislocations. The dislocation loops which are parallel to the interface and have images with lines of no contrast under two beam condition have been observed in the side of SOI Si layer. Polycrystal Si layer consists of columnar and fine grains without preferential orientation. An ultra-thin amorphous SiO2 layer exists between SOI Si layer and polycrystal Si layer.
文摘It is pointed out that crystals are discrete but not continuous materials. Hence the rotation R in decomposition F = RU and spin TY in (F) over dot F-1 ore not correct. Errors will arise in plastic deformation rare if it is directly expressed with amounts of velocity of slips in glide systems such as (gamma) over dot upsilon circle times n. The geometrical figure of crystal lattices does nor change after slips and based on this idea a simple way in mechanics of continuous media to get the plastic deformations rare induced by slips is proposed. Constitutive equations are recommended.
基金This research was supported by Russian Science Foundation under Grant 19-72-30004.
文摘In this paper,the dependencies of Young's modulus and attenuation decrement on samarium sulfide polycrystals(SmS)under various annealing temperatures are studied by the piezoelectric ultrasonic composite oscillator technique at a frequency of 100 kHz in the temperature range of 80-300 K.A decrease in Young's modulus with an increase of the annealing temperature due to the texturing of the material was revealed.At the same time,attenuation peaks were observed at temperatures about 90 and 125 K,presumably due to Niblett-Wilks and Bordoni relaxations.
基金Project(311017)supported by the Major Projects of the Ministry of EducationProjects(51175335,51305261)supported by the National Natural Science Foundation of ChinaProject(2013M530194)supported by the Postdoctoral Science Foundation of China
文摘The visco-plastic self-consistent (VPSC) model is extended to take the dynamical recrystallization (DRX) into account so that the hot extrusion texture of AZ80 magnesium alloy can be properly modeled. The effects of extrusion temperatures and imposed boundary conditions on the resulting textures were investigated, and good agreement can be found between the simulated and the measured extrusion textures. The simulated results show that the DRX grains are responsible for the formation of the {2110} fiber component since the {1010} poles of the DRX grains are tilted away from those of the unrecrystallized grains during the formation of their high angle boundaries (HABs). Furthermore, the basal poles of the grains are favorably oriented to the transversal direction (TD) where the imposed deformation is larger due to lower slip resistance of the basal slip. The elevated temperature enhances the activity of pyramidal ?c+a? slip modes and gives rise to a larger recrystallized volume fraction, resulting in a weakened extrusion texture.
基金The authors would like to acknowledge the support from the National Natural Science Foundation of China(51675331 and 51775337)Major Projects of the Ministry of Education(311017)Pei-dong WU would like to acknowledge the support from the Natural Sciences and Engineering Research Council of Canada(NSERC,RGPIN-2016-06464).
文摘The aim of present work is to develop a crystal plasticity modeling approach to integrate slip,dynamic recrystallization(DRX)and grain boundary sliding(GBS)for simulating the deformation behavior and texture evolution of magnesium alloys at high temperatures.Firstly,the deformation mechanisms of an AZ31B Mg alloy sheet at 300°C were investigated by examining texture and microstructure evolution during uniaxial tension and compression tests.DRX refines microstructure at strains less than 0.2,and subsequently GBS plays a significant role during deformation process.A GBS model is developed to evaluate strain and grain rotation induced by GBS,and implemented into the polycrystal plasticity framework VPSC.The VPSC-DRX-GBS model can well reproduce the stress−strain curves,grain size,texture evolution and significant texture differences in tension and compression tests due to GBS.The calculated GBS contribution ratio in tension is obviously higher than that in compression due to easier cavity nucleation at grain boundaries under tension loading.
基金The project supported by the National Natural Science Foundation of China(10562004)the Oversea Returning Grant of China.
文摘An orthorhombic polycrystal is an orthorhombic aggregate of tiny crystallites. In this paper, we study the effect of the crystalline mean shape on the constitutive relation of the orthorhombic polycrystal. The crystalline mean shape and the crystalline orientation arrangement are described by the crystalline shape function （CSF） and the orientation distribution function （ODF）, respectively. The CSF and the ODF are expanded as an infinite series in terms of the Wigner D-functions. The expanded coefficients of the CSF and the ODF are called the shape coefficients s^lm0 and the texture coefficients c^lmn respectively. Assuming that Ceff in the constitutive relation depends on the shape coefficients s^lm0 and the texture coefficients c^lmn by the principle of material frame-indifference we derive an analytical expression for C^eff up to terms linear in s^lmo and c^lmn and the expression would be applicable to the polycrystal whose texture is weak and whose crystalline mean shape has weak anisotropy. C^cff contains six unspecified material constants （λ, μ, c, s1, s2, s3）, five shape coefficients （s^2 00, s^2 20, s^4 00, s^4 20, s^4 40）, and three texture coefficients （c^4 99,c^4 20, c^4 40）, The results based on the perturbation approach are used to determine the five material constants approximately. We also find that the shape coefficients 2 and a s^2mo and s^4m0 are all zero if the crystalline mean shape is a cuboid. Some examples are given to compare our computational results.
